The use of neutron sources to irradiate a substance which emits radiation that is characteristic of the atoms present in the irradiated substance is a widely recognized diagnostic technique. For example, in oil logging applications, a neutron source is inserted into a hole of a geological formation. When the neutrons strike a very low mass particle, such as hydrogen in an oil deposit, they scatter and lose approximately one half of their energy. By detecting the echo levels of neutrons that return at reduced energy levels, information as to the content of the geological formation can be obtained. Although hydrocarbons reduce neutron energy levels in approximately the same manner as hydrogen in water, independent tests for the water content can also be used and combined with the neutron echo information to provide indications as to the amount of potential of oil in a formation. Uranium logging can also be conducted along similar lines wherein neutrons are directed into a formation to produce fissions in any uranium in the formation.
Conventional prior art devices for generating neutrons typically accelerate deuterium ions to 100-200 keV potentials for striking a tritium target to produce nominal 14 MeV neutrons. One type of prior art neutron generating device uses high vacuum and a plasma source triggered by passing current through a hydrided surface prior to application of an accelerating voltage. Another type of device uses a gas fill whose pressure is adjusted by means of a gas-absorbing reservoir controlled with a heater. The former design has a lifetime limited to a few hundred shots, and the latter device has a rather limited operating range and is hard to control.
Because electrons tend to be present in vacuum or low pressure gas devices that are subject to high electrical stress and are much more readily accelerated than ions, a chief design problem in ion beam generation is to reduce electron production. In conventional neutron generating devices, this is accomplished by operating at modest electric fields, so that electrons are not emitted in large quantities. In such devices, only modest ion currents can be produced, and obtaining large quantities of neutrons per pulse requires long pulses. Disadvantageously, long pulses stress the insulating envelope of a neutron tube, and some components of the electric supply current more than short pulses. Short pulses also enable more precise measurements of reflected neutrons or decay radiation and a greater time interaval is allowed for measurements.
In order to ameliorate the problems associated with high electron production, magnetically insulated diodes have been developed. When a high electrical stress is applied across a pair of diode electrodes in a vacuum, a layer of plasma is formed on the negative electrode surface, and electrons are emitted from the plasma toward the positive electrode to form an electron beam. This electron flow is controlled or inhibited in a magnetically insulated diode with the application of a magnetic field in a direction transverse to the electron flow. The electrons get trapped in the magnetic field lines and this results in the formation of an electron cloud having a strong negative charge adjacent the negative electrode.
If a proton source is provided in the vicinity of the positive electrode, the strong negative charge of the electron cloud will attract and accelerate strong proton flow. Unlike the electrons, the protons, which have masses which are approximately 1800 times that of electrons, flow through the magnetic field virtually undeflected. Ion diodes, in which positive ions are introduced in the vicinity of the positive electrode and accelerated toward the negative electrode are constructed on this principle.
In one prior art magnetically insulated ion diode a cylindrically shaped anode is concentrically disposed inside a cathode, also of cylindrical shape. A coil is positioned about the cathode to generate a magnetic field, and the entire diode is disposed inside a vacuum chamber. A laser is focused on a titanium deuteride target inside the anode, and the pulse of the laser is used to generate predominatly single-charged titanium and deuterium ions. A short time after the laser pulse, a pulse of approximately 80-150 keV is applied to the anode, and ion flow from the anode toward the cathode is generated.
The use of a laser is a highly reliable method of generating ions to be accelerated in a diode. However, the complexity of such an arrangement is impractical for many important commercial applications, such as in the oil well logging operations described above, for example. Furthermore, such prior art diodes have been relatively large and bulky. In addition, the coil used for generating the necessary magnetic field requires a bulky power supply. The insulation of leads to the solenoid and the removal of waste heat may also present formidable problems in many applications.